Photocurable compositions are well known in the art for forming printing plates and other photosensitive or radiation sensitive articles. In the field of photosensitive flexographic printing plates, the plates typically comprise a support and a photocurable surface or layer. Additional layers or surfaces on the plate include slip and release films to protect the photocurable surface. Prior to processing the plate, the release layer is removed, and the photocurable surface is exposed to radiation in an imagewise fashion. The unexposed areas of the surface are then removed in developer baths.
In the past, the unexposed areas of the solid photocurable compositions such as those disclosed in U.S. Pat. No. 2,760,863 issued to L. Plambeck, Jr. on Aug. 28, 1956, were exclusively removed with developer baths comprising organic solvents. Such solvents include tetrachloroethylene, 2-butanone, benzene, toluene, xylene, trichloroethane and solvent mixtures such as tetrachloroethylene/n-butanol. However, due to the toxicity, high volatility and low flash point of such solvents, their use gave rise to hazardous conditions and pollution problems. As a result of these problems, the field has long recognized the need to provide photocurable compositions which can be developed in non-organic solvent developing solutions, e.g. aqueous, surfactant-aqueous or alkaline-aqueous solutions. However, the printing plates resulting from these recent attempts have deficiencies in mechanical properties, e.g. flexibility (See European Application 261,910) and solvent resistance.
For instance, in addition to possessing an aqueous developable photocurable surface, a flexographic printing plate must have sufficient flexibility to wrap around a printing cylinder, yet be strong enough to withstand the rigors experienced during typical printing processes. Further, the printing plate should be soft enough to facilitate ink transfer during printing. These physical properties can be characterized generally as elongation at least 100%, preferably at least 250%, tensile strength at least 200 psi, preferably at least 300 psi and Young's modulus between about 150-750 psi, preferably about 150-450 psi, with Shore A hardness about 60 or less.
As mentioned above, previous aqueous developable compositions have not possessed all the desirable features such as flexibility, softness and solvent resistance to inks, including resistance to swell by the aqueous-based inks which are typically used in flexographic printing. For example, while the printing plate described in U.S. Pat. No. 4,023,973 issued to Imaizumi et al. on May 17, 1977, is aqueous developable, it would be deficient in softness. The photosensitive composition described therein comprises a maleic anhydride adduct of a 1,2-polybutadiene and because of the high 1,2 content of this material, i.e. 70% or more, this composition has an undesirably high rubber hardness.
European Patent Application 295,944 discloses a water-developable, photopolymerizable composition made of an ethylene/acrylic acid copolymer neutralized by a nitrogen-containing compound, reactive ethylenically unsaturated monomer and photoinitiator. This composition is disclosed to be useful with oil-based and emulsified inks, not water-based inks, for letterpress printing, a process which is a forerunner of flexography, and which uses plates that are relatively inflexible and hard. The ethylene/acrylic acid copolymer will produce a harder developed image lacking in flexibility and softness.
Other water-developable photosensitive compositions such as those which contain as the main component a high molecular weight polymer such as polyvinyl alcohol, cellulose, or the like, are also insufficient in flexibility and possess a high degree of rubber hardness and hence are unsuitable for use in flexographic printing plates.
One attempt to improve the flexibility of the photocurable layer in a flexible article such as a flexographic printing plate involves the addition of a lower molecular weight butadiene copolymer which optionally contains acrylonitrile and up to 15% by weight (0.33 ephr) carboxyl groups. See U.S. Pat. No. 4,272,608 issued to Proskow Jun. 9, 1991. In particular, Proskow discloses that by adding the low molecular weight butadiene to a high molecular weight carboxylated butadiene/acrylonitrile copolymer, a softer and more flexible composition is provided. See Column 7, lines 47-48. However, while the Proskow composition is developable in organic solvent/aqueous base mixtures, the composition still requires the use of an organic solvent for development and thus could be improved in terms of aqueous developability.
A photocurable composition described in a copending application, Ser. No. 561,849, filed Aug. 1, 1990 which is now U.S. Pat. No. 5,268,257 issued Dec. 7, 1993, for "Aqueous Developable, Photocurable Composition, Process of Making Thereof and Flexible, Photosensitive Articles Made Therefrom" describes an aqueous developable, solid printing plate which is soft, flexible and resistant to swell by aqueous inks. The composition used to make the photocurable layer on that plate comprises the reaction product of a liquid or solid carboxylated elastomer, e.g. butadiene, and a photosensitive vinyl group containing compound such as glycidyl methacrylate. The reaction product is optionally reacted with anhydride. However, even though the flexibility and solvent resistance of those photocurable compositions are sufficient, they could be improved.
Another attempt to improve the ink solvent resistance involves adding a monoolefinically unsaturated comonomer to the copolymer binder resin of a photocurable composition used to make the printing plate's photosensitive layer. See U.S. Pat. No. 4,275,142 issued to Hosaka et al. Jun. 23, 1981. Suitable monoolefinically unsaturated comonomers include (meth)acrylates, styrene acrylonitrile, etc. The copolymer binder resin used by Hosaka et al. is a copolymer of a conjugated diolefinic hydrocarbon, e.g. butadiene, and .alpha., .beta.-ethylenically unsaturated carboxylic acid. However, the copolymerization results in the ethylenically unsaturated groups of the monoethylenically unsaturated monomer being polymerized, thus preventing those groups from crosslinking with the unsaturated groups of the reactive monomers which are later added. In the present case, the inventors have found that the copolymer becomes more solvent resistant when more crosslinkable free ethylenically unsaturated groups are present on the copolymer's backbone.